Aerial photos tally more than 260 million dead trees along US coasts and Great Lakes shores

Henry Yeung and his colleagues at the University of Virginia built the count from US National Agricultural Imagery Program photographs taken between 2012 and 2023 ^[2]. They mapped dead trees at a resolution of about 1 square metre, within 10 kilometres of the Pacific, Gulf and Atlantic coasts and the US shores of the Great Lakes ^[2]. Because one image archive covers every shoreline, salty coastal stands and freshwater lakeside stands are measured the same way ^[2]. "This is the first map of ghost forests that covers the entire US," said Matthew Kirwan of the Virginia Institute of Marine Science ^[9].

Ghost forests are familiar from the US Atlantic coast, where rising seas and intense storms have driven widespread die-offs ^[19]. Waterlogged, saltier soil is estimated to have cost North Carolina 21 per cent of its coastal forest from 1985 to 2021 ^[3]. The new survey moves the question away from salt. "As a coastal scientist, I immediately think of salt as the culprit. These authors have extended it to water in general," Kirwan said ^[10].

"Inland" needs a footnote. Every tree in the count stands within 10 km of a coast or a Great Lakes shore, and forests beyond that band were not mapped ^[2]. The inland hotspots are high-elevation wetlands inside it ^[5].

The concentration is steep. If the hotspot 10 per cent is a share of forest extent, deaths per unit of forest there run at least nine times the rate across the other 90 per cent ^[20].

The tripling and the doubling are relative changes ^[6][7]. The report does not give starting rates or the number of living trees in the band, so 260 million cannot yet be stated as a share of the forest ^[1]. The causal link is the authors' suggestion: record Great Lakes levels between 2013 and 2020, driven by heavy precipitation and runoff, kept nearby soils waterlogged and raised the odds of trees dying ^[8]. Arya Iwantoro of the University of Plymouth drew a line around that. "The study provides convincing evidence that changing hydrological conditions are an important contributor to the mortality patterns, but I would be cautious about attributing every tree death directly to inundation or salinity," he said ^[11]. Storms, drought, pests, local drainage and soil conditions also interact with flooding, he added ^[12].

Both kinds of water stress have physical drivers. Global mean sea level is climbing by about 4.1 millimetres a year, and by 2100 nearly 77 per cent of the world's coastlines are expected to be affected by saltwater intrusion ^[13][14]. Each 1 degree Celsius of atmospheric warming lets the air hold about 7 per cent more water vapour, which feeds heavier rain and snow ^[15].

New Scientist reported that the finding may force a re-evaluation of how much carbon forests store ^[16]. Michelle Sims of the World Resources Institute described the downside case. "Tree mortality and decomposition releases carbon into the atmosphere, and if the forest does not recover, then these areas may not regain their capacity to sequester carbon, and it could represent a more long-term carbon stock loss," she said ^[17]. Iwantoro noted that the ecosystem replacing a forest may also accumulate carbon. "The important question is how carbon storage changes as the landscape transitions, rather than simply how much carbon is lost when trees die," he said ^[18]. I think the survey justifies re-examining the carbon held in shoreline forests, with the direction of any revision open until someone measures what grows back ^[18].